The invention is applicable to any structure to be monitored although it is particularly applicable to structures of fibre reinforced plastics composite material where impact damage, in particular, can cause a degree of damage to the structure which is difficult to detect from a visual inspection of the impact surface. Such damage is known as “barely visible impact damage” or BVID. Thus what can appear, at the impact surface, as a small chip or crack, for example, may spread out under the surface into severe damage which may compromise the structure sufficiently to cause random failure or failure under only moderate loading.
In the context of structures which are structurally critical and where impact damage is likely, such as aircraft components like wing skins, protective helmets or body armour, such potential failure under normal loading or repeat impact is unacceptable. Currently, in order to protect against such risk, regular potentially expensive inspection must be carried out. Such inspection can be extremely time-consuming, can require expensive equipment and can mean that the structure, while out of service in the case of an aircraft for example, will be costing the operator money. In the case of helmets or body armour, small impacts may go unnoticed and the article may be presumed safe and undamaged whereas a further impact may cause catastrophic damage and endanger the user's life. Such helmets may be made of glass, carbon, Kevlar or other fibre reinforced plastics composite. Body armour may be made of ceramic material or, from carbon or Kevlar composite, and airframe components may typically be made of carbon fibre composite.
US 2005/284232 A1 discloses a structure to be monitored for damage by the application of a series of conductive tracks to a surface of the structure.
The principal aim of the invention is to efficiently detect damage to a helmet, body armour or other structure which is likely to be made of fibre reinforced plastics composite material and where damage may arise from impact or excessive loading.